The layout constraints for drive trains in vehicles are increasingly restrictive due to the growing number of accessories (power steering, air conditioning, . . . ), impact standards (frontal, pedestrian, . . . ), pollution control standards (catalytic converters, particulate filters, . . . ), and alternator-starters for stop-start systems.
The hybrid automotive drive combining heat propulsion and electric propulsion offers real advantages in reducing fuel consumption, but also in reducing pollutant gas emissions (CO2 emissions).
In designing a hybrid traction drive, in addition to the usual members associated with the heat engine, at least two dedicated electrical machines must be introduced into the engine compartment in order to have enough electrical power available to meet the energy needs of the hybrid traction drive in all of the vehicle operating situations (driving conditions) where the electrical machines have to supplement the heat engine and/or recover energy.
By way of example, the following needs can be listed:                the need for a “high-power” starting member for the heat engine (for a very short, vibration-free start);        the need to be able to recharge the battery BAT while operating in electric drive mode at very low speeds or while stopped;        the need to recover energy during vehicle braking, also called “regenerative” braking;        the need to improve vehicle performance using the vehicle traction assist function, also called the “boost” drive, which provides a dynamic performance capability for the vehicle;        the need for a transmission that dynamically optimizes the best engine operating point;        . . . .        
Some auto makers meet these needs partially by offering hybrid traction drives based on transmission members with an infinitely variable transmission, also designated with the Anglo-Saxon abbreviation “IVT” (Infinitely Variable Transmission), i.e., a transmission member that can enable all the transmission ratios in a set, minimum time interval compatible with the acceleration requested by the driver.
These transmission members are chiefly composed of at least one planetary gear train and at least two electrical machines.
There are several types of hybrid traction drive architecture: a so-called “series” hybrid traction drive, a so-called “parallel” hybrid traction drive and a so-called “series/parallel” (or power splitting) traction drive.
This latter architecture with split power employs a planetary gear train coupled to the differential, and thus to the vehicle wheels, by means of a reduction gear. This architecture makes it possible to have an infinitely variable transmission (IVT) that needs no clutch.
This type of architecture is described in particular in document WO 2005/007440.
The disadvantage of this solution is that the efficiency is not always optimal because of the electrical power split needed to operate it. That is, the electrical power needed, which can be as much as 30% of the engine power, is split (or recirculates) through the two electrical machines and a power electronics unit. This creates a loss of efficiency, the overall efficiency being a product of the three electrical efficiencies.
Moreover, such an architecture requires complete industrial retooling.
Another drawback is that although they meet hybrid needs, the number of electrical machines that have to be installed in the vehicle to power the IVT (in addition to those reserved for the hybrid drive and regenerative braking) quickly becomes prohibitive.
Another solution consists in sandwiching an electrical machine between the heat engine and the gearbox, with clutch-assisted torque transition.
The disadvantage of this other solution is that firstly, it increases the length of the drive train, and secondly, having the electrical machine positioned at the gearbox input reduces the hybrid operating potential. Furthermore, this solution cannot provide high power when starting the vehicle, nor can it recharge the battery when the vehicle is stopped without penalizing starting time or efficient recharging at very low speeds. In order to have such power available, it would be required to add a stop-start-type starting system, which, firstly, is limited in power, and secondly, creates an additional encumbrance due to the width of the alternator-starter belt. As far as starting time, acoustics and vibrations, the desired performance is therefore constrained.
In addition, the electrical regenerative braking function is tricky to control, since there must not be a gear ratio shift at the same time, or there will be a torque discontinuity during deceleration. That is, in order to recover maximum energy during braking, it is required to minimize pressure drops in the heat engine and thus change the gear ratio to make the engine speed drop as much as possible.